Because he is 17.  To get the larger credit, like you received for your daughter, your child must be under 17.  You should still receive a $500 credit for him though.  Child Tax Credit

This comes as a big surprise to many parents the year their child turns 17. A child over age 16 no longer qualifies for the Child Tax credit (CTC). Although a child can still be a student dependent through age 23, and a qualifying child for EIC,  the Child Tax Credit expires the year they turn 17 and you no longer get the $2000 CTC.  Instead you will get the non-refundable (up to) $500 Other Dependent Credit.